Multi-drop communications devices are extensively used in various types of communications systems. One such type of system in common usage today is for vehicle traffic control. A number of individual locations or intersections are controlled either locally or remotely from a central control or computer. In many of these systems the interlinking of the communications equipment is by way of a type of modem that is referred to as a frequency-shift-keyed asynchronous modem which is designed to be connected to a 600 ohm voice-grade telephone type line. Where multi-point or multi-drop operations are required, a bridging amplifier is used at each interconnection or each drop. Since these systems utilize telephone lines, the lines are normally rented from the local telephone utility and there is an initial charge and a recurring charge for each of the interconnections or drops used.
Because of the recurring charges, it is sometimes more economical, in terms of total cost over a number of years, to install user-owned dedicated cable. The dedicated cable can be a pair of continuous conductors that interlink the various locations or intersections in a vehicle traffic control system. With dedicated cables, if a conventional modem as described herein is used, a means must be provided at each drop to interface the modern with the cable. If the modems are simply connected to the wire, the impedance of each of the modems causes an impedance discontinuity and results in unacceptable attenuation and reflections in the line. This is because a transmission line must be driven by, and terminated in, its characteristic impedance in order to work properly. Anywhere an impedance discontinuity exists, some of the signal power incident upon the discontinuity is reflected back up the line and is dissipated in the load causing the discontinuity, leaving only the remainder of the energy to propagate further. The power attenuation results in reduced signal levels at distant stations, and the reflections from distant stations, being replicas of earlier signals, cause distortion of the incident signal.
In order to avoid these problems, the conductors are normally connected to bridging amplifiers which then interconnect to the modems that are used at each multi-drop location. This type of an arrangement has some very distinct disadvantages. One disadvantage is the cost of providing a bridging amplifier at each multi-drop location. Secondly, and possibly one of the major disadvantages of this type of circuit arrangement, is that each of the bridging amplifiers must be powered locally. In the case of the loss of electrical power to the bridging amplifier, the bridging amplifier becomes ineffective and all of the multi-drops along the system downstream from that point are cut off to the central transmitter or receiver. This loss of control can be very serious.